It is known to locate a body catalyst within an exhaust pipe in order to clean the exhaust gas. However, it is found that more than 80% of CO, HC will be exhausted within a very short period of time before the engine and catalyst are warmed up. This resulted in a trend to develop means for rapidly heating the manifold within a limited time interval after the engine has been started.
On the other hand, there has been a proposal to improve the fuel distribution by providing a pre-mix chamber or an extension tube between the carburetor and the intake manifold, and heating the chamber or the tube from its surrounding. However, difficulties are experienced in mounting such a pre-mix chamber or extension tube on a vehicle and in heating it from the surrounding.
In a counterflow engine, a development is also in progress to provide a rapid heating of the manifold by the heat from the exhaust gas in order to improve the warming-up characteristic and reduce deleterious exhaust gas components. In a crossflow engine, the heating of the manifold takes place by utilizing the engine cooling water, but water has a relatively large specific heat, which renders a rapid heating impossible. To accommodate for such shortcoming, there have been various proposals including distributing the exhaust gas from the exhaust side to the intake side by way of a piping or feeding the exhaust gas to the intake side through a passage within the cylinder head. In either arrangement, it is found that the exhaust gas becomes cold intermediate its passage through the piping, resulting in a failure to provide a sufficient heating. In addition, the exhaust gas which was intended to provide heating retarded the warming-up of the catalytic converter and of the manifold reactor, thus degrading the exhaust gas cleaning performance. Additionally, when the temperature of the heated portion of the manifold rises, though the fuel distribution and combustion are improved, the intake air is subjected to an expansion to reduce the mass of the intake air, thus disadvantageously reducing the full output of the engine.